1. Field of the Invention
The present invention relates generally to reactive imide monomers and the radiation curable polymer systems derived from such monomers. More specifically, the imide monomers according to the present invention contain at least one ethylenically unsaturated moiety which is reactive with other ethylenically unsaturated moieties under polymerization conditions to produce a radiation curable polymer. The imide moiety becomes part of a side chain of the resultant polymer and is capable of undergoing a cross-linking reaction, specifically a 2+2 cycloaddition reaction, upon exposure to radiation to form a cross-linked polymer network.
2. State of the Art
Thermoplastic acrylic polymers and the like, which can be converted to thermosetting polymers by electromagnetic irradiation or other forms of radiation, have been known for some time. Recently, materials based on a dimethylmaleimide (DMMI) modified (meth)acrylic esters have been prepared. These systems often need to be photosensitized to achieve the proper cure because of the lack of a good chromophore. These systems are also susceptible to hydrolysis as are all esters.
Systems based on dimethylmaleimides (DMMI) are known to be photochemically reactive when irradiated with light of suitable wavelength, typically about 350-450 nanometers (a conventional photosensitizer is also often added). When photochemically reacted, the DMMIs typically dimerize in a (2+2) cycloaddition mode to give substituted cyclobutane derivatives as illustrated in Scheme I below. ##STR1##
The DMMI-monomer can be copolymerized with one or more ethylenically unsaturated comonomers in solution with radical initiators such as AIBN (Azo-bisisobutyronitrile) to yield soluble DMMI-copolymers with statistical distribution of the monomer units within the macromolecule. Once the polymer backbone is synthesized, the resulting polymer typically comprises only one readily reactive functional group, i.e., the pendant DMMI groups. These groups may be reacted together using ultraviolet light (and typically also a UV sensitizer) such as is illustrated in Scheme I, thereby cross-linking the polymers.
Such DMMI polymeric systems are generally advantageous, because the tetrasubstituted carbon-carbon double bond of the DMMI typically does not participate to any extent in the free radical polymerization reaction used to prepare the DMMI containing polymer. Most other known photosensitive functional groups will typically enter into a free radical initiated polymerization reaction, typically causing unwanted gelling and the like.
However, synthesizing the polymer backbone is generally not a simple reaction and is oftentimes difficult to perform on an industrial scale. Furthermore, the resulting polymer with DMMI functionality is generally not readily susceptible to variations in cross-linking, and oftentimes a more complex cross-linked network is necessary to achieve desired properties.
"DMMI-Photopolymers and Their Technical Application" by M. Roth and B. Muller, Ciba-Geigy AG, is a publication describing DMMI synthesis, DMMI polymer synthesis and cross-linking DMMI polymers by photoreacting the DMMI groups.
"A New Class of Photopolymers with Pendant Dimethylamaleimide Groups--Part I", Die Angewandte Makromolekular Chemie 128 (1984) 71-97 by Juergen Finter, Edward Widmer, and Hans Zweifel and "A New Class of Photopolymers with Pendant Dimethylmaleimide Groups--Part II", Die Angewandte Makromolekular Chemie 133 (1984/5) 147-170, by Juergen Finter, Zeppos Haniotis, Friedrich Lohse, Kurt Meier and Hans Zweifel, are articles which discuss and describe DMMI chemistry.
U.S. Pat. No. 4,107,174 to Baumann et al entitled "Imidyl Compounds" is directed to photoreactive imidyl compounds, such as DMMI and is also directed to polymeric imidyl compounds which are photo cross-linkable. U.S. Pat. Nos. 4,107,326 and 4,193,927 both to Baumann et al are also entitled "Imidyl Compounds" and are divisionals of U.S. Pat. No. 4,107,174.
U.S. Pat. No. 4,163,097, also to Baumann et al, discloses cross-linkable polymeric compounds which contain light-sensitive alkyl substituted cyclic imide groups.
U.S. Pat. No. 2,680,207 to Rust et al discloses N-vinyl imides containing ethylenically unsaturated groups in the imide ring which are unsubstituted. The disclosure is directed toward thermally initiated polymerization of the N-vinyl imides.
U.S. Pat. No. 4,788,295 to Rakoutz discloses only unsubstituted maleimide thermosetting reactants where reaction is initiated thermally.
U.S. Pat. No. 4,808,646 to Dahms discloses bismaleimides and maleimide acids which are unsubstituted and which are useful in preparing laminates based on bismaleimide resins.
The above-described maleimide and imidyl bifunctional monomers suffer from the fact that their radically polymerizable site is subject to cleavage either by oxidation or hydrolysis or due to the fact the unsubstituted imide groups are not easily reacted or cross-linked by radiation. Moreover, as pointed out, the resultant polymer system often requires the addition of a photosensitizer to achieve the desired cure by irradiation.